1. Field of the Invention
The present invention, in general, relates to an improved method for building a proppant plug in a horizontal wellbore at a zone of interest and, more specifically, to utilizing a fluid pill containing a high concentration of ultra lightweight proppant in order to form a proppant plug in a horizontal wellbore.
2. Description of the Related Art
New hydrocarbon reserves are increasingly being discovered in lower quality reservoirs, particularly in North America. These lower quality reservoirs require some form of “stimulation” to increase the production of hydrocarbons from wells in these fields. Fracture stimulating a well to increase the production of hydrocarbons is common practice in the oil and gas industry. Many of these reservoirs require multiple fractures to reach economic production levels and provide effective drainage. After the casing in a zone of interest has been perforated and stimulated, it must be hydraulically isolated before any new zone of interest can be exploited. A zone is often isolated by the insertion and setting of a mechanical plug, hereinafter referred to as a bridge plug, below the zone of interest.
The purpose of the bridge plug is simply to hydraulically isolate that portion of the well from a lower portion (or the rest) of the well. The isolation of the lower zone ensures high pressure fracturing fluid pumped into the well is directed to the zone of interest. The high pressure fracturing fluid is used to fracture the formation at the open perforations in the casing. The high pressure of the fracturing fluid initiates and then propagates a fracture through the formation.
In a vertical well, a bridge plug is typically run into the wellbore using a wireline, but the use of wireline to run a bridge plug in horizontal wellbores is limited to formations that are not overly sensitive to water or excess over-displacement of fluids into the fracture. This is because in order to get the bridge plug into the horizontal wellbore, the bridge plug is connected to wireline and pumped into a horizontal wellbore. The pumping of the bridge plug into the wellbore displaces the wellbore treatment fluids into the formation, which may have an adverse affect on the hydrocarbon production of the well depending on the rock formation as well as its time sensitivity to the fracture fluid. Alternatively, coiled tubing may be used to push and set the bridge plug into horizontal wellbore to isolate a zone of interest. The use of coiled tubing to run a bridge plug is time consuming and expensive because the coiled tubing needs to be removed from the wellbore between each fracturing process in order to rig up the next bridge plug that will be run for the subsequent treatment.
In an effort to reduce time and costs, another method has been developed to isolate a zone within a horizontal wellbore. This method is to build a sand plug in the wellbore at the perforation zone such that the plug hydraulically isolates the zone from the lower portion of the wellbore. To build a sand plug, the end of the fracturing fluid includes a pill of fluid containing an elevated amount of sand or proppant in comparison to the amount of sand or proppant present in the fracturing fluid. The fluid pill is pumped into the well under the fracturing pump rate. The formation at the zone of interest should have already been fractured as the fluid pill approaches the zone of interest because the fluid pill is located at the tail end of the fracturing fluid.
The pumping, and thus displacement of the fracturing fluid, is stopped as the fluid pill reaches the perforation tunnels at the zone of interest. The fluid pill with a high concentration of sand remains stationary within the wellbore with the hope that the sand or proppant remains suspended in the fluid pill. The displacement of the fracturing fluid is stopped for a period of time to allow the fractures within the formation to partially close. Once partially closed, the displacement of the fluid pill is resumed, normally at a low rate in comparison to the pump rate during the fracturing process.
The fluid pill is pumped at a low rate moving the fluid pill into the perforation tunnels and into the fractures. Typically, the pump rate is set low enough to prevent the fractures from reopening. The pumping of the fluid in the wellbore causes the fluid of the fluid pill to enter the fractures, but the high concentration of sand or proppant suspended within the fluid pill screens out against the fractures because the fractures are partially closed. Subsequently, the suspended sand in the fluid pill begins to bridge off against the fractures. As the process continues, the sand continues to pack off against the perforation tunnels and eventually the sand packs off against itself creating a sand plug in the wellbore. The slow rate of pumping is continued until the pressure within the wellbore rises indicating that a proper sand plug has been built within the wellbore.
Building a sand plug within a horizontal wellbore is a difficult process because any gravitational settling of sand or proppant in the wellbore will leave a fluid channel at the top of the hole and subsequent pumping will simply allow sand free displacement fluid to pass down the ‘channel” and into the fracture without allowing a sand plug to form. The fluid pill needs to remain stationary long enough to allow the fractures in the formation to at least partially close and so the fracturing fluid must suspend the sand or proppant for at least this period of time. If the sand does not remain suspended and settle out, it is likely that a proper sand plug will not be achieved. This is because, as the sand settles, clear fluid or fluid without suspended sand becomes located at the top of the horizontal wellbore. As pumping is resumed, the fluid of the fluid pill will simply stream over the sand bed rather than carrying the sand into the perforation tunnel because of the gap at the top of the horizontal bore.
Failing to build a sand/proppant plug will inevitably require a remedial operation involving a pump down wireline plug or a coiled tubing run.
Thus, it is critical that the sand remains suspended in the fluid pill while the fluid pill is stationary and/or being propagated adjacent the perforations. However, the sand and/or methods utilized in prior art isolation techniques have difficulty maintaining sand suspension, which leads to costly and time consuming workovers and cleaning jobs.
In light of the foregoing, it would be desirable to use an ultra lightweight proppant or neutrally buoyant proppant to build a sand plug within a wellbore. It would also be beneficial to provide a method of building a sand or proppant plug in a wellbore wherein a proppant may be used that remains suspended in a various fracturing fluids. It would also be desirable to provide a method of varying the density of proppant used in a fluid pill to build a sand plug within a wellbore as this allow a greater range of fracturing fluids that may be used in the fracturing process. It would also be desirable to provide a method of using a fluid pill containing a proppant that promotes screening out at the perforations such as using a proppant having a larger diameter than the proppant used in the fracturing process.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the issues set forth above.